This invention relates to a device for fixing a toner image on a record carrier at normal ambient temperatures. The toner image is obtained by a direct or an indirect electrostatic copying process. The device is of the type that has two unheated, rotarily supported pressure rolls having unlike diameters. The pressure roll having the smaller diameter is displaceable in the direction of the roll gap and is urged, by a spring arrangement, against the other roll.
In direct or indirect electrostatic copying process for the fixing of a developed image (consisting of an appropriate toner) on a permanent record carrier, such as different types of papers, like regular paper, laminated papers with photosemiconductor layers, transparent papers, paper and offset sheets, such as synthetic sheets as used in daylight projectors, generally two types of processes are used: The toner particles are melted by the application of heat and the carrier liquids in which the toner is suspended are vaporized. In order to accelerate the vaporization, the copy is often exposed to an air stream of ambient or elevated temperatures.
Further there are known processes which utilize heated rolls for the fixing operation.
All these processes have the disadvantage that until the copying apparatus is ready to operate, an appropriate heat-up period is necessary which may last from a few seconds to minutes. Further, in order to ensure that the heating temperature is maintained at a constant value, monitoring and control devices are necessary, which have to be adjustable, dependent upon the particular material of the copy. Further, there is a risk of overheating, for example, in case of a power failure, which may lead to the destruction of the photosemiconductor layers, particularly if selenium is used as the photosemiconductor. In wet developing processes it is of disadvantage that the copies are not always dry as they are discharged from the apparatus. Also, for the fixing with the aid of heat often a high energy input is necessary which has to be controlled by appropriate control arrangements.
If the apparatus operates with heated rolls, the latter have to be provided with coatings for preventing the melting of the toner onto the rolls and to eliminate soiling thereof. These coatings, however, have only a limited life expectancy and thus have to be replaced after a predetermined number of copying operations.
Recently, as a result of developing new toners, another fixing process has been introduced wherein the toner image is fixed without the application of heat by means of an appropriately high pressure generated by two cooperating rolls.
In order to obtain a non-smudging copy, pressures of 5 kilopond/millimeter of roll length and more are used which have to be generated by pressing the upper and the lower rolls to one another. This pressure has to be applied uniformly over the entire roll gap in order to ensure a uniform fixing.
German Laid-Open Application (Offenlegungsschrift) No. 2,341,530 discloses an arrangement in which the upper and lower rolls are pressed to one another at both ends by means of a spring-loaded bearing bracket of hinging construction. Additionally, the two rolls are slightly non-parallel so that the contact line is a large-pitch helix by virtue of an additional elastic bending deformation of both rolls. In this arrangement, however, a uniform pressure over the entire length of the helix can be obtained only if springs biased with narrow tolerances are used and further, the necessary torque is relatively high. It was further found that in case of a nonoptimal adjustment of the pressure at the contact line, the throughgoing copy tends to crease which would render it unusable.
Another apparatus is disclosed in German Laid-Open Application (Offenlegungsschrift) No. 2,353,835 which also utilizes unheated rolls. One of these unheated rolls is provided with a deformable coating which is purported to ensure a uniform pressure distribution over the roll length. It is a disadvantage of such an arrangement that in case of longer standstill periods plastic deformations may occur which do not regenerate and thus the apparatus will not operate properly. Therefore, if longer standstill periods are anticipated, the rolls have to be relieved of the pressing force; this necessitates increased expense and, if such a pressure release is not effected, malfunctions may occur.